Recent years have seen the rapid development of CCD image sensors and their present widespread use in television cameras for both amateur and professional applications. Millions of camera units, known as consumer "camcorders," have been purchased by the general public for photographing scenes of everyday life and recording the images on magnetic tape. Their small size, electrical efficiency, cost effectiveness, etc., have made CCD image sensors the imaging units of choice not only for consumer camcorders but for more critical uses where even higher picture resolution is needed. Depending on their intended uses, CCD image sensors are designed either for sequential (progressive) line-by-line readout of their image signals, or instead for "interlaced" readout of each vertical frame of first the odd-numbered lines of image signals and then the even-numbered lines in accordance with a television standard.
In order to facilitate the display of video images produced by a CCD image sensor on a standard television (TV) display, for example, the CCD image sensor is normally designed to operate in accordance with the same "standard" that the TV display uses. In the United States and a number of other countries the "standard" used for TV is that established by the national television standards committee (NTSC). In Great Britain, Germany, and certain other countries the "standard" is PAL ("phase alternation by line"), while in France and many countries in Eastern Europe the "standard" is SECAM ("sequential chrominance and memory"). While there are differences among the various standards, they all require the display of TV pictures in the form of rapidly scanned horizontal lines of vertical frames. Each vertical frame of a picture represents an instantaneous "snapshot" of the scene being imaged and the frames are displayed in rapid succession as in a motion picture. To further minimize visually apparent flicker in the displayed pictures, each frame thereof is made up of an "odd" and an "even" field superimposed on each other in rapid succession. The horizontal scan lines of an "even" field are precisely interlaced with the horizontal scan lines of an "odd" field, and so on. The NTSC "standard" specifies "525" horizontal scan lines per vertical frame, with "2621/2" lines for each of the "odd" and the "even" fields. This seemingly complicated way of displaying television images is an outgrowth of the development of commercial broadcast television over the past fifty years to the present time. However, this way has served the test of time and is not easily departed from. A much more complete discussion of television (for black and white as well as color) together with the timing, blanking, synchronizing (sync) signals, etc. required by the NTSC "standard" is given in a book entitled Basic Television and Video Systems, Bernard Grob, published by McGraw-Hill,-Inc., Fifth Edition, 1984.
Where a CCD image sensor is intended for use in digital imaging, as in the printing of color pictures for example, the CCD image sensor is normally designed for sequential, line-by-line-readout of its image signals. This sequential mode of operation is not directly compatible with the "interlaced" mode required for a standard television display.
CCD image sensors are well known in the art. Briefly described, a CCD image sensor has horizontal lines and vertical columns of light-sensing (detectors) cells closely spaced within a given area onto which an image of a scene is optically focused. By way of example, there may be hundreds of such cells in each vertical column and even more cells in each horizontal line for a total of hundreds of thousands of cells within an area which may be only a few square centimeters. Each cell represents a very small area, termed a pixel, of the total image; the more pixels present in the CCD image sensor, the finer the resolution (or apparent lack of grain) in the image reproduced by the CCD image sensor.
A CCD image sensor may have at the beginning of each horizontal line of cells a small number of cells (termed "Z ref" cells) used for determining a zero signal level. Then there are a large number of "active" cells in the line for producing pixel image signals, and finally near the end of the line there are a small number of cells (termed "D ref" cells) for determining a "dark" signal reference level, and several additional "Z ref" cells. One such CCD image sensor commercially available from the Eastman Kodak Co. has a total of 791 cells with 768 "active" cells in each horizontal line, with 9 "Z ref" cells at the beginning of the line, and following the "active" cells 12 "D ref" cells, followed by 2 "Z ref" cells at the end of the line, for a total of 791 cells. There are 484 horizontal lines of these cells arranged in vertical columns. Other CCD image sensors with fewer or greater numbers of active cells per horizontal line are similarly available commercially.
The operation of a CCD image sensor is well known in the art. The active cells of the sensor have their stored image signals (each of which corresponds to the light intensity of a small portion of an image) "read" out pixel by pixel, line by line to provide an electronic video image of a scene. Associated with each column of cells in a CCD image sensor is a separate vertical shift register. In accordance with a broadcast standard for television pictures, (for example, the NTSC "standard"), all of odd-numbered horizontal lines of an image produced by a CCD image sensor are first read out, and then all of even-numbered lines are read out, and so on. Thus at a selected instant of time, the pixel image signals then stored on the cells of the odd-numbered horizontal lines of the CCD image sensor are simultaneously shifted into respective memory positions of the vertical shift registers.
The simultaneous shifting of the multitude of individual pixel signals stored in the CCD cells of the odd-numbered lines into the respective vertical registers takes place within a short time termed "vertical blank" interval. The pixel signals thus stored in the vertical registers represent all of the horizontal lines of an "odd" field of a single frame. The pixel signals stored in all of the vertical registers are next shifted down in parallel at precise intervals within the vertical registers horizontal line by horizontal line and into respective memory positions of a line pixel register (horizontal shift register). There is a memory position in the line pixel register for each one of the vertical registers.
After a single horizontal line of pixels from the vertical registers has been shifted into the line pixel register, the image pixels of that horizontal line are clocked out of the line pixel register by a precisely numbered and spaced cycle of timing pulses (hereinafter termed "pixel clock"). The pixel image signals thus outputted from the line pixel register are applied to other circuitry, such as an analog signal processor (ASP) as is well known in the art. The number of timing pulses in a cycle of the pixel clock corresponds to the number of cells in each horizontal line of cells in the CCD image sensor. This will be explained in greater detail hereinafter.
After all of the horizontal lines of pixel image signals of an "odd" field have been shifted into and clocked out of the line pixel register, the pixel image signals stored on the cells of the even-numbered horizontal lines of the CCD image sensor are simultaneously shifted into the vertical registers and the above-described sequence is repeated line by line for an "even" field. This outputting of the "odd" and "even" fields of each frame is repeated continuously at high speed while being precisely synchronized by vertical and horizontal control signals applied to the CCD image sensor.
As is well known, a television frequency sub-carrier signal (hereinafter termed "fsc") provides for the decoding and display in proper sequence of the color-components (e.g., red, green and blue) of standard television image signals. This is also explained in detail in the above-identified book by Bernard Grob. For ease in synchronizing the outputting of the pixel image signals in each horizontal line of cells of a CCD image sensor, in accordance with a television standard, the number of cells in a horizontal line is made a convenient multiple of the frequency sub-carrier ("fsc"). This will be explained in greater detail hereinafter. For the NTSC "standard", the "fsc" is 3.5795 MHz.
The synchronizing (sync) and control signals for a standard television system (e.g., NTSC) are well suited to the needs of cathode ray tube monitors such as used in virtually all present-day television receivers. Generic standard timing generators specifically designed for producing these "standard" sync and control signals are commercially available off-the-shelf at low cost from a number of companies. However, the standard sync and control signals produced by these commercially available timing generators are not directly usable as the vertical and horizontal control signals needed for a CCD image sensor, such as described above. This is especially so where sequential, rather than "interlaced", readout of the lines of image signals is required.
A camcorder imaging system, having a CCD image sensor, typically uses a generic timing generator for generating "standard" signals to control a miniature, very inexpensive video display (with interlaced lines of "odd" and "even" fields for each picture frame) for use in a viewfinder for the camcorder. It is desirable from the standpoint of cost and convenience to be able to use such a generic standard timing generator and a miniature video display viewfinder, both of which are readily available commercially, in an imaging system where sequential readout of the lines of video signals of a CCD image sensor is required.
The present invention provides a simple, inexpensive and versatile imaging system which incorporates a television standard timing generator and a standard viewfinder display, and which provides vertical and horizontal control signals for purely sequential readout of the lines of video signals of a CCD image sensor and alternatively for viewing of the video signals in the viewfinder display.